How to proceed to UV unwrapping
It's recommended to apply the mirror modifier for humans, animals &amp; creatures. This allows you to paint both sides differently to add scars, freckles, spots, tattoos, wrinkles, skin pores, imperfections, etc for a more realistic appearance.
Hair/fur/whiskers should also have some asymentry to look natural and realistic.
You still can use mirror mode in texture paint and particle edit mode to save time.
About the displacement modifier I don't know. Do you need it? After retopology the low poly mesh should be fine.
A common workflow is to create a high poly object, retopologize it to get a low poly version. Then you bake a normal map and apply this baked texture to the low poly object. If you want to add more details like wrinkles, etc you can add a multires modifier to the lowpoly and bake the normal map again.